Method of Testing a Power Supply Controller and Structure Therefor

ABSTRACT

A power supply controller ( 20 ) is configured to operate in a test mode that facilitates measuring the value of an output signal of an error amplifier ( 36 ) of the power supply controller ( 20 ).

BACKGROUND OF THE INVENTION

The present invention relates, in general, to electronics, and moreparticularly, to methods of forming semiconductor devices and structure.

In the past, the semiconductor industry utilized various methods andstructures to build pulse width modulated (PWM) power supplycontrollers. However, it was often difficult to accurately test theaccuracy of the control loop of the power supply controller. Typically,test probes were placed in contact with various points on the powercontroller semiconductor die and were used to inject signals and samplesignals at different points on the semiconductor die. However, theseprobes had very large parasitic capacitance and inductance. When thepower supply controller was tested, the parasitic capacitance andinductance resulted in large noise signals that made it difficult toaccurately determine the value of the signals under test. In some cases,the circuit was operated at very low speed in order to reduce some ofthe noise. However, these lower speeds increased testing time therebyincreasing the cost of the power supply controllers.

Accordingly, it is desirable to have a method of testing a power supplycontroller that accurately determines the value of the signals undertest, that does not increase the test time, and that reduces the cost ofthe power supply controller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an embodiment of a portion of a powersupply system that includes a power supply controller in accordance withthe present invention; and

FIG. 2 schematically illustrates an enlarged plan view of an embodimentof a portion of a semiconductor device that includes the power supplycontroller of FIG. 1 in accordance with the present invention.

For simplicity and clarity of illustration, elements in the figures arenot necessarily to scale, and the same reference numbers in differentfigures denote the same elements. Additionally, descriptions and detailsof well-known steps and elements are omitted for simplicity of thedescription. As used herein current carrying electrode means an elementof a device that carries current through the device such as a source ora drain of an MOS transistor or an emitter or a collector of a bipolartransistor, and a control electrode means an element of the device thatcontrols current through the device such as a gate of an MOS transistoror a base of a bipolar transistor. Although the devices are explainedherein as certain N-channel or P-Channel devices, a person of ordinaryskill in the art will appreciate that complementary devices are alsopossible in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an embodiment of a portion of a powersupply system 10 that includes a power supply controller 20. System 10receives power between a power input terminal 11 and a common returnterminal 12 and responsively supplies an output voltage between avoltage output 13 and terminal 12. The voltage applied between terminals11 and 12 typically is a battery or other dc voltage. System 10 alsoincludes an energy storage inductor 14 that is coupled between output 13and an output terminal 62 of controller 20, and an energy storagecapacitor 17 that is coupled between output 13 and terminal 12.

Controller 20 receives power from terminal 11 between a voltage inputterminal 60 that is connected to terminal 11 and a voltage return 61that is connected to terminal 12. Controller 20 typically includes apulse width modulated (PWM) controller 21, a first power switch or upperpower switch 26, a first power switch or upper power switch driver 23, asecond power switch or lower power switch 27, a second power switchdriver or lower power switch driver 24, an error amplifier 36, a voltagereference generator or reference 29, a feedback circuit 53, and a testmode controller 22. Typically, the voltage applied to terminal 11 issuitable for operating controller 20. If the voltage from terminal 11 isnot suitable for operating controller 20, controller 20 may also includean internal regulator. In the preferred embodiment, switch 26 is aP-channel power MOS transistor and switch 27 is an N-channel power MOStransistor. In other embodiments switches 26 and 27 may have otherconfigurations such as a PNP and an NPN bipolar transistor,respectively. PWM controller 21 includes a clock generator or clock 31,a PWM comparator 32, and a PWM latch 33. Clock 31 generates a freerunning clock signal on a first output of clock 31 that is used to setlatch 33. Clock 31 also generates a ramp signal on a second output thatis received by comparator 32 and assists in resetting or clearing latch33. Such PWM controllers are well known to those skilled in the art.Additionally, the free running clock and the ramp signal generated byclock 31 may be stopped and reset to a low output value by negating anenable signal that is applied to an enable input of clock 31. When thereset signal is negated or low clock 31 is reset, and when the enablesignal goes high or is asserted clock 31 starts another clock cycle andthe corresponding ramp signal. Those skilled in the art recognize thatPWM controller 21 may also include other circuits and functions such ascurrent mode control or ramp compensation that is not shown in FIG. 1.Reference 29 forms a reference voltage on an output of reference 29. Thereference voltage is used to assist in regulating the value of theoutput voltage. Controller 22 is configured to control switching oralternating controller 20 between a normal operating mode in whichcontroller 20 drives inductor 14 and regulates the output voltage onoutput 13 and a test mode which is used for testing the control loop ofcontroller 20.

Controller 22 includes a first switch or test mode switch or test modetransistor 42, an impedance translator 40, an inverter 44, and a probepad 50. Probe pad 50 is formed to allow a test probe or other similardevice to contact pad 50 and to inject an external signal that switchescontroller 22 to operate controller 20 in either the test mode or thenormal mode. When the external signal is not applied to pad 50, aresistor 46 applies a high voltage to pad 50, a mode node 45, and theelements connected thereto. When a test probe is in contact with pad 50,the test probe can inject the external signal that forces pad 50 and theelements connected thereto to either a low voltage or a high voltage.The low voltage typically is substantially equal to the voltage onreturn 61 and the high voltage is sufficient to at least triggerinverter 44. The low voltage places controller 22 in the test mode ofoperation and the high voltage places controller 22 in the normal modeof operation.

Controller 20 also may include an optional transistor 43 and analternate frequency compensation network 34. Network 34 may be coupledbetween the inverting input and the output of amplifier 36 in order toprovide compensation for the poles and zeroes of the closed loop that isused to regulate the output voltage during the normal mode of operationof system 10. Such frequency compensations networks are well known tothose skilled in the art. Optional transistor 43 may be used to decouplenetwork 37 from the output of amplifier 36 during the test mode ofoperation.

In a typical system configuration, no signal is injected into pad 50 andcontroller 20 operates in the normal mode. In such a configuration,output terminal 62 typically is connected to inductor 14 and a senseinput terminal 63 of controller 20 is connected to output 13 to receivethe output voltage. In the normal mode, feedback circuit 53 receives theoutput voltage and forms a feedback signal on a feedback node 54 that isrepresentative of the output voltage. In the preferred embodiment,circuit 53 is a voltage divider that includes a first voltage dividerresistor 51 and a second voltage divider resistor 52. Circuit 53 mayhave other configurations in other embodiments. The feedback signal fromnode 54 and the reference voltage from reference 29 are received byamplifier 36 which responsively forms an error signal as the amplifieroutput signal on an output of amplifier 36. Such error signals are wellknown to those skilled in the art. In most embodiments, amplifier 36 isa transconductance amplifier that has a high output impedance. Theoutput impedance typically is greater than about one million (1M) ohmsand preferably is greater than about ten million (10M) ohms.Additionally, in most embodiments amplifier 36 has a frequencycompensation network 37 connected to the output of amplifier 36 in orderto provide a stable control loop for controller 20 and to improve thestability of the output voltage. In the preferred embodiment, frequencycompensation network 37 includes series connected capacitor 38 andresistor 39 that are connected from the output of amplifier 36 to return61. In other embodiments, network 37 may have other configurations.Since no external signal is applied to pad 50, resistor 46 applies thehigh voltage to node 45. The high voltage on node 45 enables clock 31 togenerate the free running clock signal and the corresponding rampsignal. For the case where alternate transistor 43 is used, the high onnode 45 also enables transistor 43 to couple network 37 to the output ofamplifier 36. Amplifier 36 receives the feedback signal and thereference voltage and responsively forms the error signal. The high onnode 45 also drives the output of inverter 44 low to disable transistor42. Disabling transistor 42 ensures that transistor 40 does notinterfere with the amplified error signal. PWM controller 21 receivesthe error signal and responsively generates a PWM control signal on a Qoutput of latch 33. The high voltage on node 45 also enables gates 47and 48 to receive the PWM control signal from controller 21 andresponsively generate a pair of PWM control signals that are applied todrivers 23 and 24. Drivers 23 and 24 receive the PWM control signals andresponsively generate PWM drive signals to drive switches 26 and 27.Switches 26 and 27 responsively drive inductor 14 to regulate the valueof the output voltage on output 13. Thus, the high on pad 50 and node 45enables PWM controller 21 to form the PWM control signal and enablescontroller 20 to generate PWM control signals and corresponding PWMdrive signals.

When the low voltage is applied to pad 50, for example externallyinjected into controller 22 at pad 50 by a test probe, node 45 is forcedto the low voltage. The low voltage switches controller 20 to operate inthe test mode. In most embodiments that controller 20 is operated in thetest mode, terminal 62 is not connected to inductor 14 and terminal 63is not connected to output 13 as illustrated by the dashed lines.Typically the connections to terminals 62 and 63 not formed, however, insome embodiments control switches may be used to disconnect terminals 62and 63. The low voltage on pad 50 and node 45 disables transistor 43 anddecouples network 37 from the output of amplifier 36. The low on node 45also disables clock 31 and resets the clock and ramp signals to a lowvalue that is approximately equal to the voltage on return 61. Disablingor stopping clock 31 and the corresponding clock signal reduces noisethat may be coupled into the output of amplifier 36. The low at node 45also disables gates 47 and 48 thereby preventing the output of latch 33from driving drivers 23 and 24. The low from node 45 drives the outputof gate 47 high and the output of gate 48 low which disables respectiveswitches 26 and 27 thereby inhibiting controller 20 from drivinginductor 14 and disabling controller 20 from regulating the outputvoltage. Thus, forcing node 45 low inhibits controller 20 fromgenerating PWM drive signals on the outputs of drivers 23 and 24. Thelow voltage at node 45 also drives the output of inverter 44 highenabling transistor 42.

Enabling transistor 42 couples the output signal from the output ofamplifier 36 to the input of translator 40. Translator 40 functions as avoltage follower and couples the value of the output signal minus thegate-source voltage (Vgs) of translator 40 to node 41, sense inputterminal 63, and to the input of amplifier 36 through feedback circuit53. The output signal on the output of translator 40 at a node 41 duringthe time that transistor 42 is enabled in the test mode is referred tohereinafter as a loop test signal. Since translator 40 is coupled in thevoltage follower configuration, translator 40 has a low output impedanceso that any parasitic capacitance or inductance that is connected toinput terminal 63 or to node 41, such as from test probes or testfixtures, does not inject noise into the loop test signal therebyfacilitating accurately measuring the value of the loop test signal. Theclosed loop output impedance of translator 40 in the test mode typicallyis no greater than about one hundred (100) ohms and preferably is lessthan about one (1) ohm. In the preferred embodiment, translator 40 is anN-channel MOS transistor. In other embodiments translator 40 may haveother embodiments as long as the output impedance is low and the voltagedifferential between the output signal of amplifier 36 and the loop testsignal is very predictable. For example, translator 40 may be a followeramplifier with a low offset voltage. Since the output of amplifier 36 iscoupled to sense input terminal 63 through transistor 42 and translator40, the value of the loop test signal can be measured by applying aprobe to node 41 or to terminal 63.

In the test mode, the output signal of amplifier 36 is coupled throughtransistor 42, translator 40, node 41, and circuit 53 back to amplifier36. Amplifier 36 has a high gain, typically greater than about sixtydecibels (60 db). In equilibrium the loop test signal on node 41 is afunction of the value of the reference voltage and the offset voltage ofamplifier 36 as shown by the following equation:

$\begin{matrix}{{V\; 41} = {{V\; 36} - {{Vgs}(40)}}} \\{= {{A*( {{Voffset} + {Vref} - ( {V\; 41*( {R\; {52/( {{R\; 52} + {R\; 51}} )}} )} )} )} - {{Vgs}(40)}}} \\{= {( {{A*( {{Voffset} + {Vref}} )} - {{Vgs}(40)}} )/( {1 + {A*( {R\; {52/( {{R\; 52} + {R\; 51}} )}} )}} )}}\end{matrix}$ For  A1, V 41  becomesV 41 = (Voffset + Vref) * (R 52 + R 51)/R 52

where:

-   -   V36—Output voltage of amplifier 36,    -   V41—Voltage at node 41,    -   Vgs(40)—the gate-source voltage of transistor 40    -   Voffset—the offset voltage of amplifier 36,    -   Vref—the value of the reference voltage,    -   R51—the value of resistor 51, and    -   R52—the value of resistor 52.

As shown by the equations, since the reference voltage and the offsetare amplified through amplifier 36 the value of the loop test signalincludes the value of the offset voltages of amplifier 36. For example,amplifier 36 may have an input offset voltage that is about five to ten(5-10) milli-volts. This voltage is added to the value of the referencevoltage and then amplified by amplifier 36 along with the value of thereference voltage in the manner shown by the equations. Additionally,since the output signal of translator 40 is coupled through circuit 53to the input of amplifier 36, the loop test signal includes any errorsin the value of the elements of circuit 53. In the normal operatingmode, such offset voltages and errors would detrimentally affect theoperation of the control loop of controller 20 and system 10. Therefore,it is desirable to measure the offset voltages and to minimize theeffect on the operation of system 10.

As part of the test procedure, the value of the reference voltage fromreference 29 is measured and reference 29 is trimmed or adjusted untilthe value of the reference voltage is substantially equal to the desiredvalue. This allows a guarantee of a quasi-invariant output voltage withtemperature. After reference 29 is trimmed to the desired values,controller 20 is operated in the test mode and the value of the looptest signal is measured. Since the reference voltage has beensubstantially corrected, the deviation of the loop test signal from thedesired value is primarily due to the input offset voltage of amplifier36 and the mismatch of resistors 51 and 52 as shown in the hereinbeforeequations. Thus, resistors 51 and 52 are trimmed or adjusted until thevalue of V41 is substantially equal to the desired or expected value. Insome cases, resistors 51 and 52 may be external to controller 20. Forsuch cases, amplifier 36 may be trimmed until the value of the voltageV41 is substantially equal to the desired or expected value. As is wellknown in the art there are always minor variances that prevent themeasured values from being identically equal to the desired or expectedvalues. It is well established in the art that variances of up to aboutten percent (10%) are regarded as reasonable variances from the idealgoal of exactly equal. Methods for trimming the reference voltage ofreference 29, the values of resistors 51 and 52, and the offset voltageof amplifier 36 are well known to those skilled in the art. Thus, aftercontroller 20 is adjusted in the test mode, the value of the loop testsignal on node 41 is substantially equal to the value of the referencevoltage multiplied by the ideal ratio of (R52+R51)/R52. After the testprocedure is complete and when controller 20 is operated in the normalmode, controller 20 forms an output voltage that is accurate over thedesired operating temperature range of controller 20.

In order to provide the functionality described for controller 20,regulator 28 is connected between input terminal 60 and return 61. Afirst terminal of resistor 51 is connected to both node 41 and terminal63 and a second terminal of resistor 51 is commonly connected to node54, the inverting input of amplifier 36, and to a first terminal ofresistor 52. Second terminal of resistor 52 is connected to return 61. Anon-inverting input of amplifier 36 is connected to the output ofreference 29. The output of amplifier 36 is commonly connected to asource of transistor 42, to the inverting input of comparator 32, and toa first terminal of capacitor 38. A second terminal of capacitor 38 isconnected to a first terminal of resistor 39 which has a second terminalconnected to return 61. A gate of transistor 42 is connected to anoutput of inverter 44 and a drain of transistor 42 is connected to agate of transistor 40. An input of inverter 44 is commonly connected tonode 45 the enable input of clock 31, a first terminal of resistor 46,to pad 50, and to a first input of gates 47 and 48. A second terminal ofresistor 46 is commonly connected to terminal 60 and to a drain oftranslator 40. A source of transistor 40 is connected to node 41 and toterminal 63. A first output of clock 31 is connected to the set input oflatch 33. Second output of clock 31 is connected to the non-invertinginput of comparator 32 which has an output connected to the reset inputof latch 33. A Q output of latch 33 is connected to a second input ofgates 47 and 48. An output of gate 47 is connected to an input of driver23 which has an output connected to the gate of transistor switch 26. Anoutput of gate 48 is connected to an input of driver 24 which has anoutput connected to the gate of transistor switch 27. A source oftransistor switch 26 is connected input terminal 60 and a drain oftransistor switch 26 is commonly connected to a drain of transistorswitch 27 and to output terminal 62. A source of transistor switch 27 isconnected to return 61.

FIG. 2 schematically illustrates an enlarged plan view of a portion ofan embodiment of a semiconductor device 70 that is formed on asemiconductor die 71. Controller 20 is formed on die 71. Pad 50 ispositioned in an area of die 71 that provides easy access for a testprobe. Die 71 may also include other circuits that are not shown in FIG.2 for simplicity of the drawing. Controller 20 and device 70 are formedon die 71 by semiconductor manufacturing techniques that are well knownto those skilled in the art.

It should be noted that in some embodiments, any of drivers 23 and 24,switches 26 and 27, and circuit 53 may be external to controller 20. Forthe embodiment with drivers 23 and 24 external to controller 20, gates47 and 48 may also be external to controller 20. Additionally, theoutput of translator 40 may be connected to terminal 62 instead ofterminal 63 and terminals 62 and 63 may be connected together externallyto controller 20. In some embodiments, circuit 53 may be external tocontroller 20. For such an implementation, resistors 51 and 52 should beconnected to terminal 63. Additionally, the value of resistors 51 and 52would be adjusted externally to controller 20.

In view of all of the above, it is evident that a novel device andmethod is disclosed. Coupling the output signal from an error amplifierof a PWM controller through an impedance translator facilitates testingthe value of the reference voltage and the offset voltages of the erroramplifier. The method also facilitates adjusting the PWM controller tocompensate for the offset voltage of the error amplifier and themismatch of the output resistor bridge.

While the invention is described with specific preferred embodiments, itis evident that many alternatives and variations will be apparent tothose skilled in the semiconductor arts. For example, the loop testsignal may be accessed by applying a test probe to the output oftranslator 40 instead of coupling the output of translator 40 toterminal 63. Also, instead of stopping the clock in the test mode, theoutput of amplifier 36 may be switched off to comparator 32 and theinput to comparator 32 may be switched to ground. Although controller 20is illustrated as used in a buck inductive dc-dc converter, the testmethod and structure is generally applicable other dc-dc convertersincluding boost inductive systems and capacitive charge pump systems.The word “connected” is used throughout for clarity of the description,however, it is intended to have the same meaning as the word “coupled”.Accordingly, “connected” should be interpreted as including either adirect connection or an indirect connection.

1. A method of testing a power supply controller comprising: coupling anoutput signal from an output of an error amplifier of the power supplycontroller to an impedance translator; and measuring a value of anoutput signal of the impedance translator.
 2. The method of claim 1wherein coupling the output signal from the output of the erroramplifier of the power supply controller to the impedance translatorincludes coupling the output signal from the output of the erroramplifier to a feedback circuit of the power supply controller.
 3. Themethod of claim 1 wherein coupling the output signal from the output ofthe error amplifier to the impedance translator includes translating theoutput signal from a high output impedance of the error amplifier to alow output impedance of the impedance translator.
 4. The method of claim1 wherein coupling the output signal from the output of the erroramplifier includes stopping a clock of the power supply controller fromgenerating a clock signal.
 5. The method of claim 1 wherein coupling theoutput signal from the output of the error amplifier includes inhibitingthe power supply controller from coupling a PWM drive signal to a powerswitch of the power supply controller.
 6. The method of claim 5 whereininhibiting the power supply controller from coupling the PWM drivesignal to the power switch includes inhibiting the power supplycontroller from generating PWM drive signals.
 7. The method of claim 1wherein coupling the output signal from the output of the erroramplifier includes de-coupling the output signal from a PWM controllerof the power supply controller.
 8. The method of claim 1 furtherincluding adjusting a value of the output signal of the impedancetranslator to compensate for offset errors of the error amplifier, andagain coupling the output signal from the output of the error amplifierof the power supply controller to the impedance translator, andmeasuring the value of the output signal of the impedance translator. 9.A method of forming a power supply controller comprising: configuringthe power supply controller for operation in a test mode and a normalmode for regulating a value of output voltage; configuring the powersupply controller to receive a feedback signal that is representative ofan output voltage on a first input of an error amplifier andresponsively form an output signal on an output of the error amplifierduring the normal mode; and configuring the power supply controller tooperably couple a first signal that is representative of the outputsignal of the error amplifier to an input of the error amplifier duringthe test mode.
 10. The method of claim 9 further including configuringthe power supply controller to receive a test mode signal andresponsively operate in the test mode.
 11. The method of claim 10further including configuring the power supply controller to receive thetest mode signal from a test pad internal to the power supplycontroller.
 12. The method of claim 9 further including configuring thepower supply controller to couple the output signal of the erroramplifier to a terminal of the power supply controller.
 13. The methodof claim 9 further including measuring a value of the first signal. 14.The method of claim 13 further including adjusting a value of the firstsignal and again measuring the value of the first signal.
 15. The methodof claim 9 wherein configuring the power supply controller to couple thefirst signal that is representative of the output signal of the erroramplifier to the input of the error amplifier includes configuring thepower supply controller to couple the output signal of the erroramplifier to an input of an impedance translator and to couple an outputsignal of the impedance translator to the input of the error amplifierand to not couple the output signal of the error amplifier through theimpedance translator to the input of the error amplifier in the normalmode.
 16. The method of claim 9 further including configuring the powersupply controller to inhibit generating PWM drive signals during atleast a portion of the test mode.
 17. A power supply controllercomprising: an error amplifier having a first input, a second input, andan output; and a test mode controller configured to couple the firstinput of the error amplifier to receive a signal representative of anoutput signal of the error amplifier.
 18. The power supply controller ofclaim 17 further including the test mode controller configured to couplea signal representative of the output signal of the error amplifier to aterminal of the power supply controller.
 19. The power supply controllerof claim 17 further including the test mode controller configured toinhibit the power supply controller from generating PWM drive signals.20. The power supply controller of claim 17 further including the testmode controller configured to couple an output of the error amplifier toan input of an impedance translator and to couple an output of theimpedance translator to the first input of the error amplifier.